When it comes to enhancing the performance or improving the efficiency of internal combustion engines, charging devices are known that utilize the power loss stemming from the internal combustion engine, especially the waste heat generated by the internal combustion engine. Towards this end, the charging devices are driven by the waste heat from the exhaust gas or from the coolant. For this purpose, devices are known with which the drive of the charging devices is situated directly in the coolant circuit of the internal combustion engine or directly in the exhaust-gas line of the internal combustion engine. Other drive concepts for the charging devices make use of heat exchangers in order to dissipate the heat contained in the coolant or in the exhaust gas so that it can then be employed in a circulation process to drive the charging device.
German patent specification DE 10 2007 026 869 B4 discloses a cooling unit that is used for an internal combustion engine and that has a counterclockwise thermodynamic circulation process in which the internal combustion engine is equipped with a charging device that can be connected via actuatable mechanical couplers to the internal combustion engine as well as to an electric machine and to a coolant compressor in the vapor circuit of the circulation process. Due to the evaporation of the refrigerant in the evaporator, the device is supposed to transfer more heat from the coolant circuit of the internal combustion engine into the refrigerant circuit, whereby the coolant compressor can be driven via the charging device while utilizing the waste heat from the internal combustion engine or via the separate electric machine.
When the above-mentioned device is used, however, the heat that is transferred from the coolant circuit of the internal combustion engine to the vapor circuit can be utilized for the drive of the internal combustion engine, especially in order to drive the charging device, since no expander is provided in the vapor circuit.
European patent specification EP 2 100 022 B1 discloses a charging device for charging an internal combustion engine. The exhaust-gas line of the internal combustion engine contains an exhaust-gas heat exchanger of a closed circulation system for a circulating working medium. A expansion part of the charging device is provided in the circulation system, and this part is charged with the superheated working medium supplied by the at least one exhaust-gas heat exchanger. The superheated working medium expands via the expansion part and then drives a compressor part located in the intake line of the internal combustion engine. The expansion part of the charging device is connected directly or indirectly to an electric machine via a coupler. The electric machine can be operated as a generator. Through the actuation of the electric machine, the charging pressure in the intake line of the internal combustion engine can be regulated. When the electric machine is in the generator mode, the compressor part is driven exclusively by the expansion part. Moreover, this document describes that the expansion part is connected to the compressor part via a coupler, whereby the coupler can have a clutch. The onboard network of the motor vehicle is supplied via the expansion part and via the electric machine that is directly or indirectly coupled to it.
When the above-mentioned device is used, it is not possible to recover energy during the reverse-torque operation of the internal combustion engine, for instance, when the motor vehicle is braking or coasting. Moreover, under certain conditions, the internal combustion engine cannot be charged if, for example, the battery is at a low state of charge and only a small amount of waste heat and correspondingly little or no drive output is available at the expansion part via the circulation system. A low state of charge of the battery and a low output from the vapor circuit occur when the motor vehicle is being driven mostly at low speeds, and when, from time to time, the full output of the internal combustion engine is repeatedly needed briefly, for example, in order to overtake another vehicle. Especially in the case of small internal combustion engines with a high charging level, the charging pressure has to be built up without delay so as to ensure adequate driving comfort in the motor vehicle.